1. Field of the Invention
The present invention relates to a resin composition for laser engraving, an image forming material, a relief printing plate precursor for laser engraving, a relief printing plate, and a method of producing a relief printing plate.
2. Description of the Related Art
As a method of forming a printing plate by forming concavities and convexities on a photosensitive resin layer laminated on the surface of a support, a method called “analogue plate making” is well known in which a relief forming layer that is formed using a photosensitive composition is exposed to UV-light through an original image film so as to cure selectively image portions, and then uncured portions are removed with a liquid developer.
The relief printing plate is a printing plate with a relief layer having concavities and convexities. Such a relief layer having concavities and convexities is obtained by patterning a relief forming layer containing a photosensitive composition containing as a main component an elastic polymer such as synthetic rubber, a resin such as thermoplastic resin, or a mixture of resin and plasticizer, to form concavities and convexities thereon. Among these relief printing plates, a printing plate having a soft relief layer is referred to as a flexographic printing plate in some cases.
When the relief printing plate is produced by the analogue plate making, an original image film using a silver salt material is usually required, so that cost and time for the production of the original image film are also required. Further, a chemical treatment is required for the development of the original image film, and disposal of the waste liquid of the development process is also needed. Therefore, a still simpler method of producing the printing plate such as, for example, a method using no original image film or a method requiring no development process has been studied.
In recent years, a method of plate making by scanning light exposure of a relief forming layer, in which no original image film is required, has been studied.
As a technique that does not require an original image film, a relief printing plate precursor having a laser sensitive mask layer element that is capable of forming an image mask on a relief forming layer has been proposed (for example, see Japanese Patent No. 2773847 and Japanese Patent Application Laid-Open (JP-A) No. 9-171247). According to these methods of plate making of a plate precursor, an image mask having a function similar to that of an original image film is formed from the mask layer element by laser beam irradiation in accordance with image data, so that the method is called a “Mask CTP method”, in which no original image film is needed, but in subsequent processes of plate making, light exposure using UV-light through the image mask is carried out, and uncured portions are removed by development. In view of the fact that a development process is still needed, there is room for improvement.
As a method of producing a printing plate that does not need a development process, there are many proposals about a so-called “direct engraving CTP method” in which a relief forming layer is engraved directly with a laser beam to produce a printing plate. In the direct engraving CTP method, engraving is carried out literally with a laser beam so as to form concavities and convexities that provide relief. The method has an advantage of controlling freely the shape of the relief as opposed to a method of forming relief using an original image film. Owing to this advantage, when an image like a cutout character is formed, the area thereof is engraved deeply as compared with the other areas, or in the case of a fine halftone dot image, engraving so as to form shoulders can be carried out considering the resistance to printing pressure.
However, when a relief having concavities and convexities that are resistant to printing pressure is formed on a relief forming layer that has a predetermined thickness, a high energy is required, and also, the speed of laser engraving is slow. Therefore, a problem of lower productivity as compared with a method of forming an image with a mask is brought about.
For this reason, improvement in the sensitivity of a relief printing plate precursor has been tried. For example, a flexo printing plate precursor for laser engraving that contains an elastomer foam has been proposed (see JP-A No. 2002-357907, for example). In this proposed technology, improvement in the engraving sensitivity is tried by using a low density foam for a relief forming layer. However, the resulting printing plate lacks strength because a low density material is used, thereby bringing about a problem of largely lowering the printing durability.
Further, for example, in Japanese Patent No. 2846954, and JP-A Nos. 11-338139 and 11-170718, a flexo printing plate precursor capable of being engraved with a laser beam or a flexo printing plate obtained by laser engraving is disclosed. In these documents, a flexo printing plate is obtained by mixing a monomer with an elastomer rubber as a binder, curing the resulting mixture by a heat-polymerization mechanism or a photo-polymerization mechanism, and then performing laser engraving.
As a problem of the direct engraving CTP method, slow speed in laser engraving may be mentioned. This is because the engraving depth is required to be at least 100 μm in the direct engraving CTP method in order to directly form relief, as opposed to the mask CTP method in which the thickness of the mask layer element to be ablated is about 1 μm to 10 μm.
For this reason, there are some proposals for improving the laser engraving sensitivity as described below.
For example, a flexo printing plate precursor for laser engraving that contains an elastomer foam has been proposed (for example, see JP-A No. 2000-318330). In this proposed technology, improvement in the engraving sensitivity is tried by using a low density foam. However, the resulting printing plate lacks strength because a low density material is used, thereby bringing about a problem of largely lowering the printing durability.
For example, a flexo printing plate precursor for laser engraving that contains microspheres encapsulating a hydrocarbon gas has been proposed (for example, see U.S. Patent Application Publication No. 2003/180636). In this proposed technology, improvement in the engraving sensitivity is tried by applying a system in which the gas encapsulated in the microspheres is expanded by heat generated with a laser beam and the material to be engraved is collapsed. However, a material containing gas bubbles is used, so that the strength as a printing plate is likely to be insufficient. In addition, since gas has a property of easily expanding by heat as compared with a solid, even if microspheres having a high thermal deformation initiation temperature are selected, changes in volume caused by changes in atmospheric temperature are unavoidable. For this reason, the material containing gas bubbles is not appropriate for a printing plate in which stable accuracy in thickness is required.
For example, a resin relief printing plate precursor for laser engraving that contains a polymer filler having a ceiling temperature lower than 600 K has been proposed (for example, see JP-A No. 2000-168253). In this proposed technology, improvement in the engraving sensitivity is tried by adding a polymer filler having a low depolymerization temperature. However, the polymer filler brings about unevenness on the surface of a resulting printing plate precursor, thereby providing a problem of having a significant influence on the printing quality.
As mentioned above, regarding a resin composition that can be appropriately applied to a relief forming layer of a relief printing plate precursor for laser engraving, various technologies have been proposed, but a technology having a high engraving sensitivity upon laser engraving has not yet been proposed.